Preheat oven to 350°. Cook pasta according to package directions; drain and transfer to a large bowl. Stir in sausage, soup, sauerkraut, 2 cups cheese, milk, onions, mustard and garlic., Spoon into 2 greased 8-in. square baking dishes; sprinkle with remaining cheese. Bake, uncovered, until golden brown and bubbly, 45-50 minutes. Freeze option: Cover and freeze unbaked casserole up to 3 months. Thaw in the refrigerator overnight. Remove from refrigerator 30 minutes before baking. Preheat oven to 350°. Bake, uncovered, until golden brown and bubbly, 50-55 minutes.